Abstract: Embodiments of the instant disclosure relate to novel compositions and methods for treating a subject having homocystinuria. In some embodiments, compositions and methods disclosed herein concern improving efficacy of betaine to reduce dietary compliance requirements and improve outcomes. In accordance with these embodiments, a subject having or suspected of developing homocystinuria can be treated with formate, a salt thereof, a formate derivative or formate precursor or prodrug agent alone or in combination with trimethylglycine or other HCU treatments. In other embodiments, a subject having or suspected of developing homocystinuria can be treated with zinc and/or trimethylglycine and/or formate derivative to treat homocystinuria in the subject. In other embodiments, a subject having or suspected of developing Nonketotic hyperglycinemia (NKH) can be treated with formate, a salt thereof, a formate derivative, or formate precursor or prodrug agent to treat NKH in the subject.